1. Field of the Invention
The present invention relates generally to package substrates, and more particularly to, a package substrate having landless conductive traces for electrically connecting plated through holes.
2. Description of Related Art
In conventional semiconductor package substrates applied in electronic devices, through holes such as plated through holes (PTH), vias or blind vias as disclosed by Japan Patent No. 11-008475 are formed to penetrate the substrate so as to reduce the length of the electrical conducting path to thereby improve electrical performance of highly integrated chips.
FIG. 1 is a diagram partially showing a circuit and a through hole of a conventional substrate. As shown in FIG. 1, a through hole 101 is formed to penetrate a substrate 100, and a circuit 110 is formed on a surface of the substrate 100 and the side wall of the through hole 101 by electroplating. The circuit 110 in the through hole 101 is formed with an opening 111 with a diameter of 200 μm. An insulating layer 120 is formed on the substrate 100 and the circuit 110 and filled in the opening 111. However, in the conventional fabrication process of through holes, the through hole 101 has a diameter S1 of about 300 μm and the opening 111 has a diameter S2 of about 200 μm, while the circuit 110 has a width S3 of only about 30 μm. There is a predetermined spacing between the two centers of any two adjacent through holes 101 that allows a certain number of circuits 110 to pass through. When the number of I/O connections is increased, the predetermined spacing between the two centers of any two adjacent through holes 101 is reduced, thereby reducing the number of circuits 110 allowable to pass therethrough.
To overcome the above-described drawback, as shown in FIGS. 2A and 2B, which are sectional and perspective diagrams of circuits and through holes of another conventional substrate, a through hole 201 is formed to penetrate the substrate 200, a conductive material 230 is filled in the through hole 201 and a circuit 210 is formed on the substrate 200 and the conductive material 230. Then, an insulating layer 220 is formed on the substrate 200 and the circuit 210. That is, the conductive material 230 is directly filled in the through hole 201 such that the conductive material 230 has a width equal to that of the through hole 201, which is about 200 μm. Thus, in the case the spacing between the two centers of any two adjacent through holes 201 is the same as that of the structure of FIG. 1, the number of circuits 210 allowable to pass therethrough is increased since the conductive material 230 is directly filled in the through hole 201 and has a smaller outer periphery.
However, the circuit 210 is formed by a wet etching process through a chemical reaction between the etchant of strong acid or alkali and molecules on surface of the metal layer to be etched away. Since the wet etching is an isotropic etching and requires a long-time contact of the upper surface of the conductive material 230 with the etchant, the upper surface of the conductive material 230 is apt to being over-etched and deformed, as shown in FIG. 2C. Further, in a subsequent thermal cycling test (TCT), thermal stresses are easy to generate due to high temperature environment or great variation of temperature, thereby causing crack of the contact surface 202 between the conductive material 230 and the circuit 210, as shown in FIG. 2D. In addition, as the circuit 210 has a smaller width, the crack can easily extend from one side of the contact surface 202 to the other side, which further results in separation of the circuit 210 from the conductive material 230 and thereby adversely affecting the product reliability. Furthermore, the end points of the circuit 210 is usually formed with right angles and thus stresses easily concentrate on the end points and corners 203 between the conductive material 230 and the circuit 210, as shown in FIG. 2E.
Therefore, how to provide a circuit structure for a semiconductor package substrate to overcome the above-described drawbacks has become a critical issue for the industry.